1. Field of the Invention
The present invention relates generally to electrostatic printing systems. More particularly, the invention relates to a system for electrostatically printing successive labels at spaced locations along an uncut, continuously moving strip of label stock, and for then cutting the label stock between successively printed labels.
2. Description of the Prior Art
In U.S. Pat. No. 3,787,722, there is disclosed an electrographic label printing system intended for use with stripform label stock not initially provided with either an adhesive backing or a release layer. The bare label stock is passed sequentially through a charge application station, a toning and fixing station, an adhesive application station and a cutting and ejection station. Since the disclosed imaging system appears to require that the recording medium be maintained stationary during the charge application step, a clutch and brake assembly is provided to impart an intermittent motion to the strip. Following toning, fixing and adhesive application, the strip is divided into individual labels by a pair of opposed cutting bars which are apparently intended to be operated during the intervals when the strip is stopped.
A second known type of electrostatic label printing system is disclosed in U.S. Pat. No. 3,846,150. In this patent, self-adhering label blanks are supplied in pre-cut form on a continuous carrier foil strip (consisting, for example, of impregnated paper) which is wound onto a supply roll. The label-bearing carrier strip is conveyed intermittently through an electrostatic printing station, where latent charge images are deposited on the labels, and then over a sharp bend where the adherent labels are separated from the carrier strip. The individual labels are then retrieved by a rotating member which carries each label through corona discharge, developing and fixing stations, while the bare carrier strip is simultaneously conveyed by the strip feed mechanism to a collection receptacle for subsequent reuse. The intermittent motion of the strip feed mechanism is synchronized by a photoelectric sensing system interposed across the path of the strip for sensing the position of the individual labels adhering thereto. Similar label printing systems are described in U.S. Pat. Nos. 3,861,940, 3,875,320, 3,886,563, and 3,908,191.
None of the foregoing patents addresses the problem of properly synchronizing the label printing and cutting operations when the recording medium moves continuously (rather than intermittently) through the latent imaging, developing and fixing stations. It is apparent that when the printing and cutting operations are carried out at physically separated points along the path of the recording medium, inaccurate synchronization between these operations can result in offcenter positioning of the label indicia within the borders of the final cut label, or worse, in a label which is unusable because a portion of its indicia has been cut off. In the case of a continuously moving recording medium, the cutting operation cannot be keyed to any programmed stopping of the medium in the manner apparently contemplated in the aforementioned U.S. Pat. No. 3,787,722. It is possible, of course, to avoid the cutting operation entirely by supplying the electrostatic printing system with pre-cut, self-adhering label blanks on a continuous carrier strip, as is done in the aforementioned U.S. Pat. Nos. 3,846,150, 3,861,940, 3,875,320, 3,886,563 and 3,908,191, but in that case a new supply of label blanks must be substituted each time that printed labels of a different size or shape are desired.
In prior art impression-type label printers, the problem of accurate coordination of the printing and cutting operations has been solved in various ways. In U.S. Pat. No. 3,065,691, for example, the printing type is entirely confined within a peripheral die cutter so that a properly centered label is immediately available for removal from a backing web after a combined printing and cutting operation. In U.S. Pat. No. 3,749,013, magnets are disposed along the periphery of a rotating printing drum to trip a reed switch when a complete label has been printed by the drum on a continuous label strip. Actuation of the reed switch in turn causes feeding of the label strip to cease and a knife cutter at the dispensing end of the printer to cut off a previously printed label at the appropriate point on the strip.
Neither of these approaches is applicable to electrostatic printing systems of the type contemplated in the present invention. Inasmuch as electrostatic print heads are generally designed for fairly precise dimensional tolerances with respect to the recording medium, and are often rather delicate in construction, it is impractical to combine the print head and the cutter member in one unit. Moreover, in contrast to the rotating or reciprocating mechanical apparatus used in impression printing, electrostatic print heads do not inherently require gross mechanical displacement to carry out imaging and are often made fixed and immovable. Consequently, in such systems it is not possible to synchronize the cutting operation with any repetitive physical motion of the print head.